Automatic gain control and noise suppression circuits



June 8, 1937. 2,083,243

AUTOMATIC GAIN CONTROL AND NOISE SUPPRESSION CIRCUITS o. H. SCHADE ET AL Filed April 13, .1935 2 Sheets-Sheet 1 III-- a nnuuoun v1 unnnnuo ,INVENTORS k OTTO H.8C|;ADE BY F ANCEIkfiHE ARD JR.

2 [/1 [IN- v ATTORNEY :NHHHHH 23$; i a 8& E

June 8, 1937. H, HAD ET AL 2,083,243

AUTOMATIOGAIN CONTROL AND NOISE SUPPRESSION CIRCUITS Filed April 15, "1935 2 Sheets-Sheet 2 INVENTORS OTTO HJ'CHADE 5704c HEPARD JR. 7 nu-2v L Patented June 8, 1937 UNITED STA-TS AUTOMATIC GAIN CONTROL AND NOISE SUPPRESSION CIRCUITS of Delaware Application April 13,

10 Claims.

Our present invention relates to automatic gain control circuits for radio receivers, and more particularly to receivers employing automatic volume control circuits and noise suppression, or squelch, networks.

One of the primary objects of our invention is to provide an automatic volume control circuit and squelch, or muting, network that will operate on the positive potential swing of a diode cold 10 electrode.

Another important object of the invention is to improve muting networks for receivers equipped with automatic volume control, the improvement comprising the utilization of an alternating 15 current diode rectifier whose cold electrode is empioyed to regulate the operation of a receiver network, and the rectification efficiency of thediode being dependent upon the conductivity of an automatic volume control tube whose output electrode is energized by alternating current.

Another object of our invention is to-provide an automaticgain control and noise suppression system for a receiver wherein the electrodes of the system are disposed in a common tube envelope, and the output electrodes of the gain control and muter sections are energized by alternating current.

Still other objects of our invention are to improve generally the simplicity and efficiency of as automatic gain regulation systems for radio receivers, and more especially to provide such systerns in conjunction with muter networks which are not only reliable in operation, but economically constructed and assembled in receivers.

The novel features which we believe to be characteristic of our invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by 40 reference to the following description taken in connection with the drawings in which we have indicated diagrammatically several circuit organizations whereby our invention may be carried into effect.

In the drawings:

lg. 1 shows a circuit diagram of a receiver employing the invention, I

Fig. 2 illustrates a modified form thereof.

Referring now to the accompanying drawings, wherein like reference characters in the two figures illustrate the same circuit elements, the invention is shown as applied to a conventional radio receiving system. The tube I is disposed in a tuned radio frequency amplifier having tuned input and output circuits 2 and 3. =The circuit 2 may 1935, Serial No. 16,136

be coupled to any desired source of radio frequency signals. If the set is of the superheterodyne .type, then the amplifier l is to operate at intermediate frequency and the preceding networks may comprise the usual signal collector; tunable radiofrequency amplifier; tunable local oscillator and first detector, the latter feeding its intermediate frequency output to the amplifier I through one, or more, intermediate frequency amplifiers. On the other hand if the receiver is of the tuned radio frequency type, then the amplifier l may be preceded by one, or more, stages'of tunable radio frequency amplification. The tuning condensers of the various tunable stages would then be unicontrolled in tuning. Since these various constructions are well known at the present time no further explanation is deemed necessary.

The tube 3 may be of the well known duplex diode-triode type, such as the 55 or 85 type, and includes a tuned input circuit 5 coupled to the circuit 3. The circuits 2, 3, 5 are resonated to the same frequency; and, assuming the receiver to be of the superheterodyne type, they will be fixedly tuned to the operating intermediate frequency. The diode anodes may be strapped together, and connected to the cathode through a path including circuit 5 and resistor 6 in series, the resistor being shunted by bypass condenser 1. The triode section of tube t may function as an audio amplifier, and, therefore, will have its grid connected to a desired point on the diode demodulator load resistor 5. The audio grid tap 8 is made adjustable and has audio signals impressed upon it through condenser 8; and resistor-condenser network 9, It, 9' is used to provide proper neg- I ative bias for the audio grid.

The plate circuit of thetube 4 is coupled to an additional audio amplifier tube ll through audio coupling transformer l2. The usual bypassed grid bias resistor I3 is disposed in the grounded cathode circuit of tube II; the output circuit of tube Il may be coupled to additional tubes by coupling device l4, and a reproducer may be employed after the final audio amplifier tube. In order to maintain the carrier amplitude at the circuit 5 substantially uniform over a wide range of carrier amplitude variation at the signal collector, an AVC network is used. In Fig. 1 the input for the AVG network is derived from the high alternating potential side of tuned circuit 3. While the AVG network now to be described functions to secure a result well known to those skilled v in the art, its manner of functioning and circuit elements are novel. The electrodes of the AVG tube are disposed in a tube envelope l5 which houses the electrodes of the muter, or squelch, device to be later described.

The multiple duty tube I5 comprises a pair of independent triode sections. Tubes of this type are well known to those skilled in the art of constructing electron discharge tubes, and commercial designations thereof are '79; 53; 6A4 and 19. It is believed sufficient to point out that each section of tube I5 comprises a cathode, control grid and plate. The automatic gain control section of tube I5 consists of cathode I 6, control grid I! and plate I8. The grid I1 is connected to the high alternating voltage side. of circuit 3 through condenser I9, while the cathode I6 is connected, by an adjustable tap 20, to a predeterminedpoint on the potentiometric resistor 2|. The'plate I 8 is connected to the grounded side of resistor 2| through resistor 22, the latter being the load across which the gain control bias for the controlled amplifiers is developed. The grid I I is biased negatively to a predetermined value, thus providing delayed AVC action, by grounding it through a grid leak resistor II'. Thus, the resistive portion 2I' between ground and tap 2!] provides the negative bias for grid II.

The plate I 8, is connected for energization by I alternating current from a 60 cycle current source. The latter is coupled to the system through a transformer ,whose secondary winding 23 has one terminal thereof grounded, and the other terminal thereof is connected to plate I8 through condenser 24. The AVC connection to thearnplifier. grids includes lead 25, also designatedby the letters AVC to denote that this lead applies the amplifier gain'control bias developed across resistor 22 to the grids of the controlled amplifiers. The lead 25 includes a resistor-condenser .filter network 26, the latter functioning to suppress the alternating current component of the voltage developed across resistor 22. The lead 25 is connected by a variable tap 25 to resistor 22.

The muter. section of tube I5 comprises cathode 30, control grid 3i and plate 32. The cathode 3Ilis connected to the tap28 so thatboth cathodes I6' and 30 are at a common voltage. The plate 32 is connected to the ungrounded side of 'winding 23 through a condenser 33. The grid 3| is connected to a resistor 34 through adjustable tap 35, and the resistor 34 is connected, in series with a condenser 36, between the plate I8 and ground. Thus, the resistor 34 is connected between a point of high alternating voltage and ground, since the plate I8 is connected to coil 23 through condenser 24. I

The squelching, or muting, connection is designatedby the letters N. S., and includes a lead 40 connected from the plate 32 to the low alternating potential side of the secondary of transformer I2 through a resistor 4 I. A condenser 42 is connected between ground and one side of resistor .4I, and the two elements function to suppress alternating. ripples existing in the rectified output-of the muter device. The resistor 43 is effectively. connected between the plate 32 and cathode 30 through a path including resistive portion -2I'. The direct current voltage drop across resistor 43 is used as a muting bias on the audio amplifier I I. The muter bias is superposed on thebias derived from resistor I3 in the absence of signals of a desired magnitude. When signals are received and transmitted through condenser I9, the squelch bias across resistor 43 is removed and the audio amplifier is operative,

up on' the plate I8.

Of course, the bias resistor I furnishes the normal radio frequency amplifier bias; as signals increase in magnitude the bias superimposed through lead 25 increases to reduce the gain of the controlled amplifiers. The need for the muting network arises by virtue of the maximum sensitivity of the amplifier .I when weak signals are receivedj noises being unduly amplified during such reception the muting network is used to render reproduction inefficient. This is, also, true for inter-station positions of the tuning means, since carrier intensity at such tuning positions is a minimum and well below the noise level.

The functions of the circuit elements associated with tube I5 will now be described in detail. It is first pointed out that the alternating current operated direct current amplifier shown herein is generally disclosed and claimed in application Serial No. 727,968, filed May 28, 1934 by Francis E. Shepard, Jr. In the present case we have disclosed an application of the aforesaid type of amplifier forobtaining an amplified automatic volume control voltage, and a voltage to bias one of the audio amplifiers below cut-off for squelching the receiver audio output when the received carrier intensity decreases below a predetermined intensity level. i

V The grid I! of the AVG section of tube I5 is normallybiased by means of a direct current voltage to a point greater than that at which the plate current is cut off. As a result there can be no rectified direct current potential built As a consequence of the latter action the AVG voltage is zero in the absence of signals, and, furthermore, the direct current voltage on the grid 3I of the muter section of tube I 5 is at its least negative value. Condenser 36 presents an infinite impedance to direct current, and, therefore, has no shunting or dividing effect in applying the negative direct potential on plate I8 to grid 3I.

The tap 35 on resistor 34 is adjusted so that the positive peaks of alternating current applied to the grid 3| are suflicient to swing the grid above its cut-off value with respect to its cathode. causes the muter section of tube I5 to rectify when its plate is swung positive on the positive peaks of the 60 cycle alternating current. Hence, a negative direct current potential is built up on the plate 32, and this is employed, after adequate filtering through network 4I--42, to bias the signal input grid of audio tube II to cut-off. The two diodes in the tube I5 operate on the same half cycle.

In other words, in the absence of signals transmitted through condenser I9, there is no automatic volume control bias impressed upon the signal grid of amplifier I through lead 25. Additionally, the muter section of tube I5 impresses the cut-off bias through lead 40 upon the audio amplifier II, and thereby successfully prevents the transmission of undesired noises to the reproducer of the system. The noises, of course, are created by virtue of the maximum sensitivity of the radio frequency amplifiers of the receiver preceding the demodulator input circuit 5. These noises occur whenever the received signal carrier amplitude .decreases below a predetermined inten'sity'level, and also when the receiver is tuned between desired stations.

When signals are received, and the grid II is'swung less negative on the radio frequency voltage peaks from the amplifier I, the AVG section of tube I5 will become conductive. As a result rectified direct current potential, negative This with respect to ground, will be built up on the plate l8. After filtering through network 26, this rectified potential is used as an amplified AVC voltage, and the latter voltage reduces'the gain of the controlled amplifiers in such a manner that the carrier amplitude at the input circuit 5 of the demodulator is maintained substantially uniform'over a wide range of received carrier amplitude variation. The rectified direct current potential built up on plate l8, when signals are received, is also fed to the grid 3! of the muter section of tube Hi. This negative potential impressed on grid 3| reduces the conductivity of the muter section of tube I5 to a point such that the path between plate 32 and cathode 30 becomes non-conductive. Consequently there is no rectified direct current potential produced on plate 32, and the muter, or squelch, voltage is removed from the signal grid of audio tube II.

The delay action of the automatic volume control can be adjusted by means of the initial direct current grid bias impressed on grid ll. This is accomplished by varying the position of tap 20 on resistor 2|. The signal intensity at which the muter action, or squelch, becomes operative can be predetermined by adjusting the alternating current potential on the grid 3|, and this is accomplished by means of the tap 35 which is slidable over resistor 34.

While the circuit arrangement shown in Fig. 1 shows the tube [5 performing its functions in response to the direct impression of signal energy through condenser is upon grid H, the same functions can be secured by means of the impression of a direct current potential upon the grid ll. Thus, in Fig. 2 there is shown a system which is similar in all respects to the arrangement shown in Fig, l, with the exception that there is impressed upon the grid ll of the AVG section of tube l5, a direct current potential derived from the demodulator load resistor 6. The grid IT is connected by a lead to a desired negative point on load resistor 6, and the lead 4i) terminates in an adjustable tap which is adjusted to the diode plate side of the resistor 6.

In this way the grid II is made less negative by the direct current voltage developed across the diode resistor 6, and the amplitude of the direct current voltage derived across resistor 6 is dependent upon the signal carrier amplitude at circuit 5. The operation of the system shown in Fig. 2 is precisely the same as that described in connection with Fig. 1, and for this reason it is not believed necessary to repeat the description of the operation of the system. A separate diode demodulator is used, with its cathode connected to the high alternating voltage side of input network 5. The audio grid of tube 4' is connected to resistor 6 for audio signals. The negative bias for the audio grid is secured in the same manner as shown in Fig. 1.

While we have indicated and described several systems for carrying our invention into effect, it will be apparent to one skilled in the art that our invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of our invention, as set forth in the appended claims.

We claim:

1. In combination with a high frequency amplifier, a demodulator and an audio amplifier, a pair of rectifier circuits, the output electrode of one rectifier being connected to a gain control electrode of said first amplifier to vary the gain thereof, the output electrode of the second rectifier being connected to a gain control electrode of the audio amplifier to regulate the gain thereof, a source of alternating current, means impressing current from said source upon said rectifier circuits for rectification, means responsive to received signal amplitude variations to vary the operation of the one rectifier, means for regulating the operation of the second rectifier with the direct current voltage output of the first rectifier.

2. In combination with a high frequency amplifier, a demodulator and an audio amplifier, a pair of rectifier circuits, the output electrode of one rectifier being connected to a gain control electrode of said first amplifier to vary the gain thereof, the output electrode of the second rectifier being connected to a gain control electrode of the audio amplifier to regulate the gain there of, a source of alternating current, means impressing current from said source upon said rectifier circuits for rectification, means responsive to received signal amplitude variations to vary the operation of the one rectifier, means for regulating the operation of the second rectifier with the direct current voltage output of the first rectifier, the electrodes of said two rectifiers being disposed in a common tube envelope, and the space current paths of the rectifiers being in parallel. I

3. In combination with a high frequency amplifier, a demodulator and an audio amplifier, a pair of rectifier circuits, the output electrode of one rectifier being connected to a gain control electrode of said first amplifier to vary the gain thereof, the output electrode of the second rec tifier being connected to a gain control electrode of the audio amplifier to regulate the gain thereof, a source of alternating current, means impressing current from said source upon said rectifier circuits for rectification, means responsive to received signal amplitude variations to vary the operation of the one rectifier, means for regu lating the operation of the second rectifier with the direct current voltage output of the first rectifier, said signal responsive means comprising a signal transmission path between said first rectifier and said high frequency amplifier.

4. In combination with a high frequency amplifier, a demodulator and an audio amplifier, a

pair of rectifier circuits, the output electrode of one rectifier being connected to a gain control electrode of said first amplifier to vary the gain thereof, the output electrode of the second rectifier being connected to a gain control electrode of the audio amplifier to regulate the gain thereof, a source of alternating current, means impressing current from said source upon said rectifier circuits for rectification, means responsive to received signal amplitude variations to vary the operation of the one rectifier, means for regu lating the operation of the second rectifier with the direct current voltage output of the first rectifier, said signal responsive means comprising a direct current connection between a conductivity control electrode of the first rectifier and a direct current voltage point in the demodulator.

5. In combination with a high frequency amplifier, a demodulator, and an audio amplifier, a pair of rectifier circuits, the output electrode of one rectifier being connected to a gain control electrode of said first amplifier to vary the gain thereof, the output electrode of the second rectifier being connected to a gain control electrode of the audio amplifier to regulate the gain thereof, a source of alternating current, means impressing current from said source upon said rectifier circuits for rectification, means responsive to received signal amplitude variations to vary the operation of the one rectifier, means for regulating the operation of the second rectifier with the direct current voltage output of the first rectifier, said energizing source being of an audible frequency, and the electrodes of both rectifiers being housed in a common tube envelope.

6. In combination with a high frequency amplifier, a demodulator and an audio amplifier, a pair of rectifier circuits, the output electrode of one rectifier being connected to a gain control electrode of said first amplifier to vary the gain thereof, the output electrode of the second rectifier being connected to a gain control electrode of the audio amplifier to regulate the gain thereof, a source of alternating current, means impressing current from said source upon said rectifier circuits for rectification, means responsive to receive signal amplitude variations to vary the rectification of the one rectifier, means for regulating the rectification of the second rectifier with the direct current voltage output of the first rectifier, means operatively associated with the first rectifier to adjust the rectification point thereof. '7. In combination with a high frequency amplifier, a demodulator and an audio amplifier, a pair of rectifier circuits, the output electrode of one rectifier being connected to a gain control electrode of said first amplifier to vary the gain thereof, the output electrode of the second rectifier being connected to a gain control electrode of the audio amplifier to regulate the gain thereof, a source of alternating current, means impressing current from said source upon said rec- 40 tifier circuits for rectification, means responsive to received signal amplitude variations to vary the rectifier operation of the one rectifier, means for regulating the rectifier operation of the sec- 0nd rectifier with the direct current voltage output of the first rectifier, means operatively associated with the second rectifier to adjust the signal level at which it rectifies said current.

8. In a radio receiver of the type including a super-audible frequency transmission network, a

no demodulator and an audible frequency transmission network, a tube provided with a pair of parallel rectifier sections, means for impressing alternating current energy of an audible frequency mission, and means'responsive to the operation of the first section for regulating the rectification efficiency of the second section.

9. In combination with at least two cascaded signal transmission tubes, a source of alternating current of a frequency substantially less than the signal frequency, means for rectifying current from said source, means for automatically varying the gain of the first of said tubes inversely with respect to amplitude variations in the rectified output of the rectifying means, signal responsive means for controlling the rectification efficiency of the rectifying means in the same sense as variations in signal amplitude, a second means for rectifying current from said source, means for varying the gain of the second of said tubes in the same sense as the signal amplitude variations, and means, responsive to a predetermined increase in the rectified output of the first rectifying means, for decreasing the rectification efficiency of the second rectifying means.

10. In combination with at least two cascaded signal transmission tubes, a source of alternating current of a frequency substantially less than the signal frequency, means for rectifying current from said source, means for automatically varying the gain of the first of said tubes inversely with respect to amplitude variations in the rectified output of the rectifying means, signal responsive means for controlling the rectification efficiency of the rectifying means in the same sense as variations in signal amplitude, a second means for rectifying current from said source, means for varying the gain of the second of said tubes in the same sense as the signal amplitude variations, and means, responsive to a predetermined increase in the rectified output of the first rectifying means, for decreasing the rectification efllciency of the second rectifying means, and additional means for adjusting the signal amplitude levels at which said two rectifying means become operative.

O'I'IO H. SCHADE. FRANCIS H. SHEPARD, JR. 

